The present invention relates to a power amplifier suitable for an audio amplifier.
As audio power amplifiers, Class A or Class B SEPP (signal-ended push-pull) power amplifiers are generally used. Although the Class A SEPP power amplifier generates less distortion, because output transistors thereof require a relatively large idling current the amount of heat generated increases as the output power decreases. At the maximum output power level, the power dissipation is generated as much as the output signal power. On the other hand, as the Class B SEPP power amplifier uses no idling current, both the cross-over distortion and the switching distortion take place, while the efficiency thereof is much higher than that of the Class A SEPP power amplifier. Still, the efficiency of the Class B SEPP power amplifier at the maximum output power level reaches at most 78%, and at smaller output power levels the efficiency thereof more decreases.
In the laid-open Japanese patent publication No. 94162/78 there is disclosed a power amplifier which is much higher in efficiency than the Class B power amplifier. This power amplifier is provided with an emitter follower transistor having its base connected to receive an audio input signal. The collector voltage and the emitter voltage (terminal voltage of a load, that is, an output voltage) of the emitter follower transistor are compared by a voltage comparator having a hysteresis characteristic. This voltage comparator controls a switching element which supplies the collector of the emitter follower transistor with a power supply voltage. Between the switching element and the emitter follower transistor a smoothing circuit is connected.
This power amplifier achieves a higher efficiency than the Class B power amplifier by making the collector voltage of the emitter follower transistor follow the waveform of the input signal. It is, however, indispensable for this power amplifier to have an emitter follower transistor. If the high frequency characteristic of the emitter follower transistor is poor, the waveform of the emitter voltage or the output voltage of the emitter follower transistor is delayed from the waveform of input signal voltage and the waveform of the collector voltage is further delayed, thereby causing generation of distortion.
In the aforementioned prior art amplifier, it is impossible to compensate the waveform delay by preparing a delay circuit within the main path of the input signal to the emitter follower transistor.